1. Field of the Invention
The present invention relates to a two-dimensional noise reducer for video signals.
2. Description of the Related Art
Methods of reducing noise in video pictures are classified as two-dimensional if they use only the two spatial dimensions, and as three-dimensional if they also use the time dimension.
Three-dimensional noise reduction works well for stationary parts of a video picture. It is relatively easy to store one or more preceding frames in a frame buffer and detect noise in stationary parts of the current frame by comparing these parts with the preceding frames. For moving parts of the picture, however, it is necessary to add motion compensation by detecting motion vectors. This leads to complex algorithms, requiring extra hardware, and works well only under limited conditions. Particularly for pictures with high-speed motion, detecting the motion and finding the correct motion vectors remains a computationally challenging task.
A simpler method of noise reduction is noise coring: the removal of high-frequency signal components of low amplitude. This method can be practiced in two dimensions, making it applicable to moving pictures without the need for motion compensation, regardless of the motion speed. Noise coring has the drawback, however, of degrading the picture by removing low-amplitude fine detail as well as noise.
Another two-dimensional method, disclosed in Japanese Patent Application Publication No. 8-214192, processes each picture element (pixel) individually by averaging the pixel value with the values of other pixels located in directions in which there is no pixel with a greatly different value. That is, a pixel is averaged with other pixels located in all directions that show a certain degree of correlation with the pixel. This method turns out to be unreliable because of false correlation detection, leading to the inclusion of irrelevant pixels in the average value.
Despite the difficulties associated with motion compensation, a satisfactory alternative has yet to be found. There is an unmet need for a reliable two-dimensional noise reduction method that can avoid the loss of detail caused by noise coring.